Our intent in the program in ciliate genetics is to develop species of the genus Tetrahymena into more effective instruments in the study of basic eukaryotic mechanisms. We are particularly interested in the heterogeneity of breeding systems and life styles that have evolved on the basis of a common body design. Tetrahymenine ciliates consist of dozens of species that range from asexual amicronucleates through autogamates to conjugating species with sophisticated and diverse systems for regulating mating. They also range from apparently free-living bacteriophagous forms through carnivores to parasites on a number of invertebrate hosts. An understanding of the diversity of these protozoa requires an exploration of their relationships. In this connection we propose to continue our analyses of ribosomal RNA sequences. We are testing the validity of a "noise filter" based on "shared ditypic sites" that promises to make even short RNA sequences informative about evolutionary events at remote times. This approach we illustrate with sequences from 5S, 5.8S, 17S and 23S rRNAs. We propose the extension of this evolutionary analysis to several dozen strains collected in recent years that have not been assigned to any of the named species. We have only begun to work with the parasitic/commensal and carnivorous forms. Beyond that we plan to place the tetrahymenines securely in ciliate evolutionary history by continuing our study of general ciliate phylogeny. We expect to identify where and when the ciliated protozoa emerged from the eukaryotic roots, and we hope to discover something of the nature of that radiation. Our breeding studies will concentrate on T. pigmentosa and T. malaccensis, which are evolutionary distinct from the more commonly studied T. thermophila, and for which we have a substantial body of unpublished observations. These species show some strikingly different expressions of mating types and serotypes. We suspect that the superficial similarities among the tetrahymenines obscure some interesting differences in the management of nucleic sequences in the somatic macronucleus.